make netinet/in.h suppress clashing definitions from kernel headers

the linux kernel uapi headers provide their own definitions of the
structures from netinet/in.h, resulting in errors when a program
includes both the standard libc header and one or more of the
networking-related kernel headers that pull in the kernel definitions.

as before, we do not attempt to support the case where kernel headers
are included before the libc ones, since the kernel definitions may
have subtly incorrect types, namespace violations, etc. however, we
can easily support the inclusion of the kernel headers after the libc
ones, since the kernel headers provide a public interface for
suppressing their definitions. this patch adds the necessary macro
definitions for such suppression.

include/netinet/in.h

@@ -385,6 +385,21 @@ struct ip6_mtuinfo {
 
 #define IPV6_RTHDR_TYPE_0       0
 
+#define __UAPI_DEF_IN_ADDR      0
+#define __UAPI_DEF_IN_IPPROTO   0
+#define __UAPI_DEF_IN_PKTINFO   0
+#define __UAPI_DEF_IP_MREQ      0
+#define __UAPI_DEF_SOCKADDR_IN  0
+#define __UAPI_DEF_IN_CLASS     0
+#define __UAPI_DEF_IN6_ADDR     0
+#define __UAPI_DEF_IN6_ADDR_ALT 0
+#define __UAPI_DEF_SOCKADDR_IN6 0
+#define __UAPI_DEF_IPV6_MREQ    0
+#define __UAPI_DEF_IPPROTO_V6   0
+#define __UAPI_DEF_IPV6_OPTIONS 0
+#define __UAPI_DEF_IN6_PKTINFO  0
+#define __UAPI_DEF_IP6_MTUINFO  0
+
 #ifdef __cplusplus
 }
 #endif